The invention relates to a rectifier device for a three-phase dynamo of a motor vehicle. Three equal, mutually independent windings u, v, w, spatially staggered by 120xc2x0 each time, are present according to the classic principle of the three-phase dynamo (stator). The exciter winding is present on the rotary part in the interior (rotor). The moment an exciter current flows through the windings, a magnetic field will arise which in its turn will induce a three-phase AC voltage in the stator windings which supplies the three-phase current when a load is connected. This current is rectified by means of a bridge rectifier circuit and serves to supply the load and to charge the battery. The output voltage of the dynamo is controlled by means of a controller (pulse width modulation of the exciter current: PWM), i.e. the average value of the exciter current is adjusted in dependence on the instantaneous available battery voltage.
Known electrical rectifier arrangements for motor vehicle applications are built up with diodes in special housings, which diodes are mechanically pressed in into metal cooling plates (so-called press-fit diodes) or are soldered onto cooling plates. The construction of the electrical and thermal connection between the diodes and a cooling body (usually a cooling plate which also serves as the component carrier) is of special importance because the power losses in the components in question are to be removed with sufficient speed on account of the high electric current values (typically approximately 50 A to 130 A). The mechanical connection of the (usually eight) diodes to one another and the electrical connections are usually formed by a pressed metal screen molded around with synthetic resin.
A very good thermal connection of the diodes to the cooling plate can be achieved through soldering. Diodes in housings are surrounded by a plasticized synthetic resin which is decomposed above a certain temperature. To comply with the requirements set by the high operating temperature of a rectifier, a soft solder with a higher melting temperature is to be used for the connection. The melting point of the solders suitable for this lies above the decomposition temperature of the synthetic resin of the encapsulated diodes. This means that diodes in housings can only be used for rectifiers when the requirements are reduced. Known rectifier arrangements accordingly show no encapsulation and anchoring of the rectifier diodes.
It is accordingly an object of the invention to provide a rectifier device for three-phase dynamos for motor vehicles which offers the diodes a better protection.
This object is achieved by means of a rectifier device with at least one cooling body, with at least one diode arranged on a cooling body, with at least one protective layer arranged on the diode and on the cooling body, and with at least one receptacle at the cooling body for connection to the protective layer. The rectifier device according to the invention renders possible the use of diode chips without housings which are not surrounded with synthetic resin until after soldering. The synthetic resin of the housing thus is not subjected to a treatment which lies above its decomposition temperature. The receptacle at the cooling body may be formed, for example, by a depression with an undercut. However, many fastening possibilities are conceivable which offer a simple and permanent connection or adhesion between the protective layer and the cooling body. The aspect of thermal loading is to be given special attention here. A rectifier arrangement according to the invention utilizes the cooling plate of the rectifier as a constructional part for the diode housing. The diodes are soldered on the cooling plates and encapsulated thereon afterwards. The thermal interconnection is thus optimized and the number of solder spots is reduced to a minimum. This enhances the reliability and useful life of the rectifier. The constructional incorporation of the semiconductor diodes of the rectifier into the total construction is the major feature of the invention. In general, however, the invention may be utilized for all applications with power semiconductors.
An embodiment of the invention will now be explained in more detail below with reference to drawings, in which